Conventional digital transmission networks are based on a plesiochronous digital hierarchy (PDH). A problem with plesiochronous digital networks is that it is not possible to locate in a bit stream of a higher-level system a given primary-rate signal, but the higher-level signal has to be demultiplexed through each intermediate stage down to the primary rate level. For this reason it has so far been expensive to build branching connections that need several multiplexers and demultiplexers.
The synchronous digital hierarchy (SDH) was developed in order to eliminate problems associated with the conventional digital transmission networks. The SDH facilitates the introduction of ever higher transmission rates, at the same time making the network maintenance and administration easier and more flexible than in prior-art networks. The SDH is based on the STM (Synchronous Transport Module) frame where all lower-level transmission rates are time-division multiplexed into a higher-rate frame structure. This arrangement has the advantage that the location of each signal belonging to said STM frame is known and, thereby, every individual signal in an STM frame can be extracted from the frame.
The basic structure of an SDH network is ring-like, where a ring comprises a plurality of network elements. Ring-like basic structures can be joined together by interconnecting them at a network element called a digital crossconnect (DXC). In an SDH ring, the network elements that are not interconnected with other ring networks are called add-drop multiplexers (ADM). These network elements may be assigned various tasks such as monitoring duties.
As was already mentioned, the synchronous digital hierarchy is based on the STM frame. The STM frame can be used at different transmission rates so that we have an STM-1, STM-4 or STM-16 frame depending on the transmission rate, or generally, an STM-N frame, where N stands for the multiple of the primary rate. FIG. 1 shows the basic structure of the STM-N frame. The STM-N frame comprises a payload and section overhead (SOH) bytes. Rows 1 to 3 and 5 to 9 in columns 1 to 9×N are reserved for the SOH bytes. Row 4 in columns 1 to 9×N is reserved for administrative unit (AU) pointers. The other columns, i.e. bytes, are reserved for the payload of the STM-N frame.
The payload can be used in many ways; for example, signals of different rates can be transported in an STM frame by dividing the payload of the STM frame into tributary units of different sizes that have address and payload information of their own. Typically, the payload of an STM-1 frame comprises 63 so-called VC-12 frames, each of which has a transmission capacity of 2048 kbit/s. The payload of each VC-12 frame typically comprises 30 or 31 channels of 64 kbit/s. Each 64 kbit/s channel typically serves one phone call. The location of every signal in the STM frame is known so that any STM frame signal can be easily either added to or removed from the STM frame.
An SDH frame comprises several different smaller units. The administrative unit is a smaller unit used for path- and section-level adaptation. It comprises a payload, or a virtual container (VC) and the aforementioned AU pointer that indicates the phase difference of the payload and the STM-N frame. One or more administrative units with a fixed position in the STM payload form an administrative unit group (AUG). The virtual container mentioned above is a multiplexing element that supports path-level connections in the SDH. A characteristic of the VC frame is that it transports untouched in the synchronous transmission network up to the destination of the container. A virtual container comprises a payload and path overhead (POH). The synchronous payload portion of the virtual container is called a container (C). A container comprises a payload signal the frequency of which may be equalized in order to synchronize it with the STM frame. A tributary unit (TU) forms a path between the higher and lower levels. It comprises a payload, or VC frame and a pointer. The pointer can be used to indicate the position of the payload in the TU frame. One or more tributary units with a fixed position in the VC frame form a tributary unit group. Said tributary unit group is formed by multiplexing the TU frames with each other.
Simply put, the compilation of an STM frame can be described as follows: First, the payload signal is inserted in the container. The container may comprise a plurality of tributary units or it may be a broadband signal. Next, the lower-level VC frame is formed by adding the overhead bytes into the container. Tributary units, or TU frames, are formed by adding the pointer bytes into the VC frames. Tributary units are multiplexed into tributary unit groups. By further multiplexing the tributary unit groups a higher-level VC frame is formed which is aligned with an AU frame which is further multiplexed into an administrative unit group. When section overhead is added to the administrative unit group, an STM frame results.
An SDH transmission network is divided into three layers: the regenerator section (RS), multiplexer section (MS), and the path (P). The aforementioned overheads are used in the management of the connections of said layers. The regenerator section lies between a line terminal and a regenerator or between two regenerators. A line terminal refers to a means with which a standard electric signal is converted into a corresponding line signal suitable for the transmission path used. A regenerator is used to amplify the signal attenuated on the transmission path. The multiplexer section lies between two multiplexers, and a path refers to an interval between points, where the frame is compiled and resolved.
An STM frame must include information indicating the network element the STM frame is directed to. At each SDH level there is a certain byte in the frame that gives the address information for that frame. The byte is located at a certain position in the frame structure of each level. The address information bytes of the different SDH levels can be called trail trace identifiers (TTI) in general. At the highest level, i.e. RS layer, the TTI in the section overhead field of the STM-1 frame is the byte J0, which is given as an example in FIG. 2. The figure shows other bytes, too, which are not discussed in this context. At the next levels S4 and S3, the TTI in the over-head field is J1. In the overhead fields of the lower levels S2 and S12 the TTI is J2.
All TTI identifiers mentioned above advantageously comprise 16 bytes. A basic structure of said TTI identifiers is shown in FIG. 3. The first byte in the TTI is the frame start symbol. The first byte in the TTI is recognized in the sequence by setting the most significant bit to one. In the other bytes of the identifier the most significant bit is set to zero. Together the first bits of the bytes can be said to form a frame alignment signal (FAS). The other bits of the first byte in the TTI comprise an error checking code calculated using a known error checking method such as cyclic redundancy check (CRC), for example. The bits marked x in FIG. 3 constitute the actual 15×7-bit address information known as access point identifier (APId) by means of which the network elements recognize the frames addressed to it.
In accordance with standard ETS 300 417-1-1 the TTI identifier is utilized as shown in FIG. 4. A TTI identifier RxTI is read from the received signal and compared with the expected TTI identifier value ExTI. If the identifiers compared are identical, an accepted trail trace identifier signal, AcTI, is issued. But if the signals are not identical, a detected trace identifier mismatch signal, dTIM, is issued. In that case, an alarm indication signal, AIS, is then advantageously generated at the output.
Let us next consider possible error conditions that might occur in the network. Connections are usually backed up such that a special back-up arrangement is provided for each connection, meaning that each data path has an alternative data path which will be used when a disturbance occurs on the primary data path. Then the network element that detects the disturbance switches the connection to the alternative data path. In such a case it may happen that there are various delays in the alternative data path more than in the primary data path so that the frame alignment is shifted as shown in FIG. 5. The letter A in FIG. 5 means that the frame “jumps” out of frame alignment because of the disturbance. Another disturbance might occur upon transition from a first network operator's network to a second network operator's network, say between two different countries, in which case the clock synchronizations in the two operators' networks could differ from one another. In that case the receiving network operator attempts to take the payload in buffer circuits, but in some cases the buffer capacity runs out, resulting in an overflow and frame misalignment. In these and other similar error conditions the STM frame alignment is lost, whereby a network element on the data path switches an alarm indication signal, AIS, into the signal to indicate the loss of frame alignment. When the STM frame alignment is lost, the alignments of the tributary units and VC frames will be lost, too. When these alignments are lost, the TTI information, address information APId, and the AcTI signal will be lost as well.
When frame alignment is lost, it has to be regained in order to obtain the address information from the frames. Frame alignment is sought for using said frame alignment signal, for example. When STM frame alignment is caught, VC frames can be searched for by interpreting the pointers. When the STM frame alignment is found, the TTI, i.e. byte J0, of that particular level can be found. The frame alignment having been found, the AU-4 pointers in the administrative unit are interpreted in order to find the VC-4 frame which further reveals the TTI identifier J1. After that it is possible to find the lower-level tributary unit TU which further reveals the lower-level VC frames. When the lower-level VC frames are found, the TTI identifiers J2 can be found, too. When all the TTI identifiers have been found, said access point identifier, APId, can be checked. If the TTI identifiers prove correct, the TTI identifier information can be accepted by issuing the accepted trail trace identifier signal AcTI.
Loss of frame alignment e.g. from reasons mentioned above and the regaining of frame alignment may take a long time. The more time is consumed the more frames and, hence, data are lost. For example, the loss and regaining of frame alignment plus the time without frame alignment may add up to some tens of milliseconds at the STM frame level. During this time many frames, each lasting 125 μs, will be lost and, thereby, a lot of data. During said time when frame alignment is lost, alarm indication signal AIS is advantageously added to the signal. Time consumed in the loss and regaining of frame alignment is multiplied when going to the lower levels.